Always show the add domain button - redirect to payment page if user hasn't paid.
On the desktop app - when you skip adding payment options, the add domain button doesn't show anywhere and the documentation says to add a domain, press the green button (which is assuming you've added payment). Now when I go to account options > payment there is a message in small print that says "All you need to do is add a card. This allows you to create custom domains and send to anyone." I think it would be much better UX to have the green domain button always show and redirect to this payment page if you haven't paid as I got completely stuck setting up my account in this way.